The present invention relates to a process for the preparation of 2-mercaptobenzothiazole from aniline, carbon disulfide and sulfur.
2-Mercaptobenzothiazole (MBT) is a starting material which is industrially very important for the preparation of vulcanization accelerators such as, for example, dibenzothiazolyl disulfide and sulphenamides.
Industrially, MBT is generally produced by the Kelly process (U.S. Pat. No. 1,631,871), wherein aniline, carbon disulfide and sulfur are reacted under elevated pressure at elevated temperatures. The resulting crude MBT must then be purified to remove unreacted starting compounds, intermediates and products from secondary reactions. The MBT purification, which is customary industrially, in principle consists of a reprecipitation, in which the crude MBT is dissolved in sodium hydroxide solution and the tar-like by-products are decanted off, filtered off or extracted. The aqueous sodium MBT solution is subjected to a further oxidative treatment, if appropriate; the MBT is then precipitated using sulfuric acid and filtered off (cf. German Patent 2,258,484).
The disadvantages of a purification of this type are that the aqueous phase contains about 85 kg of Na.sub.2 SO.sub.4 per 100 kg of MBT formed, which has to be disposed of with the effluent. In addition, about 15 kg of waste products (tars) are produced per 100 kg of MBT and the bulk of these waste products has to be disposed of as solid waste, for example by means of combustion. A considerable proportion of the by-products also passes into the effluent, where it causes a high chemical oxygen demand. The chemical oxygen demand (COD) is determined in mg/l O.sub.2 by back-titration of dichromate. The analytical methods vary and must be indicated by the result.
In addition, in the case of modern catalytic oxidation processes using oxygen to form MBTS and sulfenamides (compare German Pat. No. 3,113,298 and, respectively, German Pat. No. 3,325,724) a high purity of the MBT is required because it is then possible to work with very low catalyst concentrations. Such purity is not guaranteed with conventional production methods for MBT.
Those skilled in the art have therefore made many attempts to prepare MBT both in high purity and in high yield. U.S. Pat. No. 3,031,073 describes another process for the preparation of MBT, in which aniline, carbon disulfide and sulfur are heated under pressure in a cyclic procedure, the pressure is reduced, the MBT is removed from the crude product, the residue is mixed with the necessary amount of aniline, carbon disulfide and sulfur and this mixture is again heated to form MBT. Any method is said to be suitable for isolation of MBT from the crude reaction product. This patent specification points out in detail a purification method using a water/carbon disulfide emulsion, which incidentally also contains a surface-active substance.
However, according to Example 1 of the said U.S. Pat. No. 3,031,073, it is also possible to use carbon disulfide on its own as a purifying agent. After a reaction time of 5 hours at 245.degree. C., the reaction is discontinued by releasing the pressure and removing the reaction mixture from the autoclave. With this procedure the pressure generated by hydrogen sulfide is eliminated by lowering the pressure to atmospheric pressure. In order to purify the crude product, the latter is mixed with carbon disulfide in an autoclave and the mixture is heated to 140.degree. C. and kept at this temperature for 30 to 45 minutes. Finally, based on the purification method, MBT is obtained in a yield of 94% and in a purity which is indicated as 99.5%. When this procedure was repeated, a total yield of 82% was obtained. The MBT had a purity of 98%.
DE-OS (German Published Application) 2,652,394 describes a process for the purification of crude MBT using carbon disulfide. According to Example 1, aniline, carbon disulfide and sulfur are allowed to react for 50 minutes at 220.degree. C. and the hydrogen sulfide is then removed. For purification, the liquid MBT is brought into contact with cold carbon disulfide, with the formation of two phases, a slurry of MBT and carbon disulfide being formed. The MBT is said to have a purity of 99.5%, which merely in view of the quoted melting range of 172.degree. to 175.degree. C. (literature value for pure MBT: 180.degree.-182.degree. C.) appears dubious. The yield was about 20% and is thus entirely uneconomical.
The purity of the MBT has an effect during the conversion to the said vulcanization accelerators. Experiments in this regard give the following findings: in oxidation reactions in accordance with German Patent No. 3,113,298 and German Patent No. 3,325,724 (preparation of MBTS and respectively, CBS or TBBS), MBT prepared and purified in accordance with U.S. Pat. No. 3,031,073 and obtained after three process cycles led to dark greasy deposits on the stirrer shaft and reactor wall. These deposits bound the catalyst and severely retarded the reaction.
There was therefore a need for an economic process for the preparation of a highly pure MBT in high yield. The recycling of intermediate and by-products should not have an adverse effect on the quality of the MBT such that oily deposits are formed in the oxidation reaction to give dibenzothiazolyl disulfide or sulfenamides and the reaction is substantially slowed down, which is reflected in a low space-time yield.